ASHRAE Journal - June 2021 - 47

COLUMN ENGINEER'S NOTEBOOK
The architect had designed an entrance that consisted
of glass walls and glass roof (Figure 6). The glass was a
laminated monolithic structural blast-resistant glass. But
this had the trade-off of having subpar thermal performance.
The design winter outside air
temperature in St. Louis was 7.5°F
(-13.6°C). We wanted to maintain
inside conditions of 72°F (22°C) at
50% RH to protect the museum displays.
That is a dew point of 52.4°F
(11.3°C). We chose to be conservative
and assess the glass surface temperature
with an outside air temperature
of 2°F (-17°C).
We found that the glass inside surface
temperature would be below 30°F (-1.1°C). This
was obviously a problem since our dew point was 52.4°F
(11.3°C). We determined that condensate would form
on the glass when the outside air temperature dropped
below 20°F (-6.7°C). There are a lot of hours below 20°F
(-6.7°C) each year in St. Louis. Condensation and frost
would form and obstruct the view. Condensation could
also drip on visitors from the glass roof. We had to fi nd a
way to mitigate the condensation and frost.
After assessing several options, including infrared
heaters and electric heat wire embedded in the glass,
we decided on a two-prong approach. First, we could
lower the inside dew point by placing the most sensitive
artifacts inside sealed enclosures with their climate
controlled by microclimate generators. This allowed us
to lower the RH in the museum in the winter. Now we
would maintain a minimum inside RH of 36% at 72°F
(22°C). That lowered the dew point to 43.6°F (6.4°C).
Then we chose to deliver hot air to the glass surface via
fl oor grilles.
Upon CFD analysis we found that the glass inside surface
temperature would be raised enough to prevent condensation
in all but the most extreme weather conditions.
We had mitigated the risk of condensation enough that
the project team was willing to accept the minimal risk.
If this same building were located in Ontario, we
would not have needed to provide hot air at the glass
or add the climate-controlled enclosures because of
Ontario's winter design temperature of 38.4°F (3.6°C).
Summary
Designers should pay close attention to a project's
J U N E 2 0 2 1 ashrae .o rg ASHRAE JOURNAL
47
FIGURE 6 Museum of Westward Expansion.
climate. The amount of moisture in the air depends on
the project location and time of year. Designers must
consider outside conditions during all seasons and their
impact on the operation of the building and HVAC systems.
What is appropriate in one
climate does not necessarily make
sense in other climates. Especially
for fi rms that share work between
offi ces in different climate zones, it
is important to educate employees
about this.
Acknowledgments
We want to thank this column's
reviewers: David Sellers, P.E.,
Member ASHRAE, Facility Dynamics; John Bade,
Associate Member ASHRAE, Osorio; Nathan Ho, P.E.,
Member ASHRAE, P2S; Erin McConahey, P.E., Fellow
ASHRAE, Arup; Peter Andrisevic, Associate Member
ASHRAE, Trane; Armin Hauer, Member ASHRAE, ebmpapst.
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ASHRAE Journal - June 2021

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